The present invention relates to heat treating aluminum base alloys prior to hot working operations. The invention is particularly concerned with a homogenization heat treatment of the aluminum base alloys prior to extrusion.
The metal working process known as extrusion involves pressing metal stock through a die opening having a predetermined configuration in order to form a shape having indefinite length and a substantially constant cross section. In the direct extrusion process with which this invention is particularly concerned, the aluminum base alloy stock is preheated and placed in a cylinder which is also usually heated. The heat treatment process of the present invention may also be utilized prior to indirect extrusion processes. The cylinder utilized in the direct extrusion process has a suitable die at one end and a reciprocable piston or ram having approximately the same cross sectional dimensions as the bore of the cylinder. This piston or ram moves against the stock to compress the stock and cause the metal to flow through the die opening. The pressure exerted on the stock during this operation raises the internal temperature of the stock as a result of redundant work within the metal stock body.
The present invention is particularly concerned with aluminum base alloys of the aluminum-magnesium type. Extruded profiles of aluminum-magnesium alloys have considerable commercial value. Such alloys find diversified use as structural materials because of their very high strength to weight properties. In order to produce extruded articles from such alloys in the most economical manner, the extrusion process should be carried out at the highest extrusion speed possible for the apparatus being used. The aluminum-magnesium alloys have been found difficult to hot work in commercial production. These difficulties have been manifest as pronounced edge cracking, alligatoring or surface cracking during hot working, or as pronounced surface cracking or break up of the extruded material during the extrusion process.
Extrusion speed and temperature are factors which affect the quality of aluminum-magnesium alloys as extruded products. In order to achieve acceptable surface quality in the extruded products, a certain limited range of extrusion speeds and temperatures must be closely observed with the range being related to the size of the extrusion and the reduction in cross sectional area of the metal stock during the extrusion process. Exceeding the predetermined speed and temperature ranges generally causes a rupture of the extrusion surface as indicated above and also other defects which result in rejection of the extruded product.
A limiting factor for extrusion of an aluminum alloy is the onset at some extrusion speed of the phenomenon known as surface checking or chatter cracks. These are surface defects which form a pattern of fine transverse cracks resulting from longitudinal tensile stresses which are high compared with the strength of the alloy at its working temperature. The incipient cracks may be no deeper than 0.001 to 0.005 inch but they are unacceptable from the standpoints of surface appearance, finishing ability, dimensional accuracy and mechanical integrity of the extruded product. It is known that this surface checking or chatter cracking phenomenon occurs at lower speeds as the extrusion temperature is increased. It is also known that high strength alloys must be extruded more slowly and at lower temperatures than lower strength alloys in order to avoid cracking of the high strength alloys. This suggests that there is an interaction between the flow and fracture mechanisms during hot working of the alloys. There is also a direct relationship between the hot ductility and the extrudability of the alloys.